The present invention relates to a hydraulically operated underreamer.
This is a tool that is used to enlarge boreholes. Such tools can be used in drilling oil, gas, water and, in mining, drilling of construction holes and wells and also in the formation of shotholes for blasting. An underreamer has two operative states, one closed or collapsed state where the diameter of the tool is sufficiently small to allow movement of the tool in the narrowest part of the borehole, and one opened or partly expanded state where one or more toolholders (arms) with cutters on the ends thereof pivot out from the body of the tool. In this position the borehole is enlarged as the tool is rotated and lowered.
A drilling type underreamer usually is used in conjunction with a drill bit below the underreamer. The drill bit forms the hole to be underreamed at the same time as the underreamer enlarges the hole formed by the bit. Circulation of drilling fluid must be provided to the drill bit to remove cuttings during the drilling operation.
Underreamers of this type usually have hinged arms (toolholders) that have a tendency to break during the drilling operation and must be fished-up or withdrawn from the borehole. The tool has pockets where the arms are situated in the closed state. These pockets have a tendency to be filled with materials from the drilling operation, which makes collapsing of the arms difficult, thereby providing a substantial chance that the underreamer will become caught or hooked in the borehole, and this will lead to severe problems when attempting to remove the tool. Costs also can be considerable. In addition, this type of reamer is very large and heavy and has a complicated structure composed of many parts. Such type of underreamer is, for example, described in U.S. Pat. No. 4,282,941.